The present invention relates to a composition for stripping gold coatings from copper substrates. More particularly, the invention is concerned with a stripping composition and method which will rapidly remove gold that has been deposited by electroplating or chemical plating techniques on copper substrates and which will not attack the copper substrate.
In the plating art various compositions have been proposed for stripping gold metal coatings or deposits from metal substrates. In general, these prior art methods for stripping the gold involve the use of chemical formulations containing a soluble cyanide, e.g. sodium or potassium cyanide, a ring-substituted nitrobenzene acid compound as an oxidizing agent, and a small amount of a lithium, lead or bismuth compound for accelerating the stripping action. It has been recognized that the use of an aqueous solution from the aforementioned chemical composition can effectively strip gold and gold alloys off nickel, nickel alloy, and Kovar substrates by immersion with a minimum or substantially no attack on the metal substrate. However, it also has been appreciated that when the same solutions are used for stripping gole deposits from a copper substrate severe attack of the copper substrate results.
The recent Solidum et al. U.S. Pat. No. 3,935,005, which issued Jan. 27, 1976, provides the background as well as the problems associated with the removal of deposited gold metal from copper substrates. In this patent the reasons for the need for such stripping solutions in the plating industry is delineated along with discussions of known stripping solutions. The prior patents to Springer et al. U.S. Pat. No. 2,649,361 and Grunwald U.S. Pat. No. 3,242,090 are discussed by Solidum. Referring first to Springer, it is noted that this patent discloses a stripping composition comprising sodium cyanide, sodium nitrobenzene sulphonate, and sodium hydroxide. Grunwald, on the other hand, discloses a stripping composition comprising a complex metal tetracyanide salt, a nitro substituted aromatic compound such as nitrobenzene sulphonic acid, nitrobenzoic acid, nitrochlorobenzene, nitrophenol and nitrobenzaldehyde. The complex salt component of the Grunwald composition is formed from a metal selected from the group consisting of zinc, cadmium and copper.
As noted by Solidum, the compositions of the Springer and Grunwald patents have been extensively utilized as strippers of gold from various types of substrates. Nevertheless, two major deficiencies of such stripping compositions were found. Firstly, the stripping compositions have not removed the metal deposits as rapidly as might be commercially desirable. Secondly, these compositions have a tendency to attack various metal substrates such as copper.
The invention of Solidum was to overcome, inter alia, these deficiences. It is directed to a dry powder stripping composition for addition to water and to form thereby, an aqueous solution for stripping gold and silver deposits from a variety of substrates. The composition disclosed by Solidum calls for the presence of a major proportion of a soluble cyanide compound selected from the group consisting of alkali metal cyanide, ammonium cyanide and mixtures thereof. Other components in the stripping composition include a soluble lithium compound; a soluble compound of a metal ion selected from the group consisting of lead, bismuth, and mixtures thereof; and a ringsubstituted nitrobenzene acid compound. The latter is selected from the group consisting of alkali metal and ammonium salts of methyl-nitrobenzoic acid, chloro-nitrobenzene sulfonamide, amino nitrobenzoic acid, and chloronitrobenzoic acid.
Although the Solidum stripping composition is stated to rapidly strip gold and silver from such substrates as stainless steel, nickel, molybdenum and alloys of iron, nickel cobalt and manganese; there still is a problem when copper is the substrate. More particularly, Solidum states that with a copper substrate the period of immersion has to be closely controlled since the stripping composition will attack the copper substrate.